1. Field of the Invention
The field of the invention is a method and apparatus for making a rapid, relatively accurate measurement of the hematocrit of blood.
2. Description of the Prior Art
Heretofore various techniques have been used to measure hematocrit of blood. Hematocrit is the volume of red blood cells in the blood expressed as a percent of volume of red blood cells per 100 milliliters of whole blood. With one technique, a quantity of whole blood is diluted and the red blood cells are counted either by placing a quantity of diluted blood on a slide and counting with a microscope or by using optical counters which measure light scattering of red blood cells as the highly diluted blood is passed through a tubing traversing a light path from a light source to a scattered light detecting photodetector.
In another technique, the blood is centrifuged and the amount of packed red blood cells that have been centrifuged is measured to determine hematocrit.
Although a relatively high degree of accuracy can be obtained using these known techniques for measuring hematocrit, these techniques are very time consuming. Accordingly, it is desirable to provide a rapid means for making a relatively accurate measurement of hematocrit.
The optical density of a blood solution is not linear with the concentration of particles therein. Since hematocrit is a measure of the concentration of particles, specifically red blood cells, in blood, attempts to utilize the optical density or light transmission through a blood sample for measuring concentration of particles have not heretofore been successful. In this respect, optical density and light transmission measurements are used mainly to measure concentration in a solution rather than concentration of discreet particles. Also, discreet particles behave differently in solution since the particles have a tendency to reflect light off the surfaces thereof and often do not permit the light to pass through them.
Also, when the light is reflected, it bounces from one particle surface to another particle surface and such reflected light can make its way through the solution without ever going through a particle. For these reasons, optical density or light transmission measurements are not usually utilized for making measurements of particulate matter in a solution.
In the past however, various methods and apparatus have been proposed for utilizing light in making measurements of various characteristics or blood. Examples of such prior art methods and apparatus for making various analyses of blood, primarily of highly diluted blood, utilizing optical techniques are disclosed in the following U.S. Patents.
______________________________________ U.S. PATENT NO. PATENTEE ______________________________________ 3,123,066 Brumley 3,692,410 Jurany et al. 3,893,767 Fulwyler et al. 3,905,769 Carroll et al. ______________________________________
Moreover, an optical system for measuring hematocrit heretofore has been proposed in U.S. Pat. No. 3,830,569. In this system a laser light is passed through successive samples of blood containing hemolyzed and spherized red blood cells. According to the teachings of this patent, the blood is highly diluted and the light is utilized to enumerate the number of red cells, white cells and platelets in a sample for computing hematocrit. As will be explained in greater detail hereinafter, the method and apparatus of the present invention differ from the method and apparatus disclosed in this patent in that, according to the present invention, a rapid measurement of hematocrit is made by measuring the bulk optical density of blood when an intense, coherent light is passed through the blood. Also, the method and apparatus of the present invention is a non-invasive system which makes measurements exterior of a sterile blood conducting conduit.
Another optical system for measuring hematocrit is disclosed in U.S. Pat. No. 3,923,397. According to the teachings of this patent, hematocrit is measured by passing monochromatic light through a highly diluted blood sample and measuring a characteristic representative of the amount of energy absorbed by the sample. Then, the hematocrit ratio is computed by applying a proportionality factor to the measurements of transmitted light energy by the following expression: EQU H=K(2-log.sub.10 T)
where:
H=hematocrit sought to be measured,
T=measured transmitted light energy,
and
K=the proportionality factor.
Again, the method and apparatus of the present invention differ from this previously proposed method and apparatus by the measurement of bulk optical density to obtain a rapid, relatively accurate measurement of hematocrit and by the utilization of a linear relationship and proportionality factor derived from a linear curve fit for measurements of the amount of laser light transmitted through, and the hematocrit of the blood through which the laser light is passed, over a given hematocrit range.